An increase in cardiac work evokes a rapid increase in mitochondrial oxygen consumption and ATP synthesis to meet the energetic demand. The mechanism of this response is thought to involve stimulation of oxidative phosphorylation at multiple sites in the pathway. Two commonly proposed mechanisms include i) enhancement of the rate of mitochondrial electron transport and ATP synthesis by the products of ATP consumption (ADP, Pi), and ii) upstream stimulation of NADH production by Ca2+ at the level of the Krebs cycle. In intact cardiac muscle, ischemia and reperfusion alters both ion homeostasis and mitochondrial function, but the effects on global mitochondrial control have not been investigated. The goal of this project is to characterize the role of intracellular cations in the mechanism of energy supply and demand matching in intact cardiac muscle and isolated cardiomyocytes and to determine how these processes are modified by ischemia and reperfusion. Specifically, we will employ novel fluorescence techniques to examine how the interdependent actions of intracellular Ca2+, Na , and K+ modulate the mitochondrial bioenergetic response to a change in workload in normal and post-ischemic cardiac muscles and cells. The dynamics of Ca2+ transport from the cytoplasmic space to the mitochondrial matrix during cardiac excitation-contraction coupling will be measured, and the coupling of mitochondrial Ca2+ influx to the stimulation of energy metabolism will be determined. We will test the hypothesis that intracellular Na+, which increases dramatically during ischemia, plays a key role in modifying the bioenergetic response to changes in intracellular Ca2+. We will also examine whether mitochondrial Ca2+-activated K+ (mitoKca) channels are activated in response to physiological changes in mitochondrial matrix Ca2+, thus modifying the energetic response. This data will be integrated into a newly developed comprehensive computational model of the cardiac cell as a framework for understanding the effects of altered ion homeostasis and ischemia on the electrophysiology, force production, and Ca2+ handling properties of intact cardiac muscle. The results will be compared with the proteomic and biochemical analyses, and metabolic control experiments to be carried out using the same animal model in the other components of the Program Project. Elucidating the contribution of ion homeostasis to supply and demand matching will permit us to rationally design therapeutic strategies for coping with contractile failure and arrhythmogenesis in the post-ischemic heart.